1. Field of the Invention
This invention relates to a CRT display which produces picture images on a cathode ray tube, and more particularly to a CRT display which exhibits an improved color purity.
2. Description of the Prior Art
Shown by way of example in FIG. 1 is a fragmentary sectional view of a shadow mask type color CRT display (hereinafter referred to simply as "CRT" for brevity) which appeared in "TV TECHNOLOGY", pp. 43-50, June 1990. In this figure, indicated at 1 is a CRT, at 2 is a panel of a plate-like shape, and at 3 is a funnel of a funnel-like shape. The panel 2 and funnel 3 are integrally formed of glass to constitute an envelope of CRT 1.
Indicated at 4 is an electron gun which is located within the envelope at a neck portion of the funnel 3, and at 5 is a shadow mask located within the envelope along the panel 2. Denoted at 6 is fluorescent material of three primary colors coated on the inner surface of the panel 2 to emit blue, green or red light.
The reference 7 indicates an electron beam which is produced by the electron gun 4 to excite a corresponding one of the fluorescent materials of the three colors. Designated at 8 is a deflection yoke for scanning each electron beam on and along the fluorescent material 6 of the corresponding color.
In operation, the shadow mask which is generally referred to as a color-selecting electrode containing a multitude of perforations functions in such a way as to let each of the electron beams for the respective colors reach only the fluorescent material of the corresponding color while blocking the electron beam for the fluorescent material of the other colors.
With regard to the material of the shadow mask itself, it has been the general practice to employ a metal in consideration of the etching process usually resorted to for forming the perforations, the workability into a desired shape, the function as an anode, etc.
As mentioned above, the shadow mask 5 plays the role of blocking electron beams 7, so that its temperature is elevated by the impinging energy of the electron beams 7. The temperature elevation gives rise to a problem of thermal expansion because metal is used for the shadow mask as mentioned before. More specifically, the shadow mask which is generally formed in a spherical shape undergoes thermal deformation as indicated at 5a and 5b in FIG. 5, namely, from a mask shape 5a at a low electron beam level to a mask shape 5b at a high electron beam level.
This phenomenon in which the shadow mask is expanded toward the panel 2 as a result of the impingement of electron beams is called "doming" in the art. In this regard, FIG. 4 illustrates major portions of FIG. 3 on an enlarged scale. The positional relations between the fluorescent material 6 and an electron beam 7 before and after the doming are discussed below with reference to FIG. 5.
When the screen luminosity is low due to a low electron beam level, the shadow mask is in a state as shown at 5a in FIG. 4. Accordingly, the center of the electron beam 7 correctly hits the center of the fluorescent material 6. This state is illustrated in FIG. 5(A). As seen in FIG. 2, the shadow mask 5 is initially set in a predetermined position 5a which is determined such that the center of the electron beam 7 from a beam outlet for red color of the electron gun 4 hits the center of the red fluorescent material 6.
As the screen luminosity becomes higher with an increasing electron beam level, the doming phenomenon occurs to the shadow mask as a result of its temperature elevation, shifting the shadow mask 5 to the position indicated at 5b in FIG. 5. Consequently, the center of the electron beam 7 is deviated from the center of the fluorescent material 6. This state is illustrated in FIG. 5(B). As seen in FIG. 5, as a result of the positional deviation of the shadow mask 5 from 5a to 5b, the tracks of the electron beam 7 are shifted parallelly inward, making it difficult for the electron beam 7 to hit the fluorescent material correctly and exciting the fluorescent material in inwardly waned condition in microscopic observation.
FIG. 5 illustrates microscopically observed positional relations (deviations), showing that the actual position of the electron beam 7 is shifted inward relative to the fluorescent material 6 as a result of the doming phenomenon.
In this way, the doming drops the luminosity in peripheral areas to impair the uniformity across the whole screen area. Therefore, attempts have been made to suppress the temperature elevation of the shadow mask by putting a carbon graphite film on the inner surface of the panel 2 or by putting a bismuth oxide film on the inner surface of the shadow mask, or to suppress the thermal deformation of the shadow mask 5 by employing as its material an Invar material (a nickel-iron alloy) of low thermal expansion (with a thermal expansion coefficient of about 1.2.times.10.sup.-6 /.degree. C.) in place of iron (with a thermal expansion coefficient of about 12.times.10.sup.-6 /.degree. C.) which has thus far been generally adopted.
On the other hand, there have been strong demands in the market for suppression of leakage of magnetic fields from CRT displays, particularly, magnetic fields of 1 kHz to 400 kHz. To comply with these demands, it has become a usual practice to mount compensation coils 9 over and under the deflection yoke 8 as shown in FIG. 6.
The horizontal deflection current or part of the horizontal deflection current is passed through these paired compensation coils 9 thereby to produce magnetic fields (compensation magnetic fields), which act to bend the tracks of the electron beam 7, impinging the electron beam 7 in outwardly shifted positions relative to the fluorescent material 6 in microscopic observation as shown in FIG. 7(A). This is because the tracks of the electron beam 7 are deviated only in the horizontal direction by the magnetic fields produced by the horizontal deflection current flowing through the compensation coils 9.
Generally, the mount position of the deflection yoke 8 in the axial direction of CRT is set at a reference position 10 as shown in FIG. 8(B). In this state, satisfactory color purity is obtained as long as the center of the electron beam 7 is in alignment with the center of the fluorescent material 6 as shown in FIG. 9(B). However, if the mount position of the deflection yoke 8 is shifted toward the panel 2 from the reference position 10 as shown in FIG. 8(A), the electron beam 7 is shifted inward relative to the fluorescent material 6 as shown in FIG. 9(A), putting the fluorescent material 6 in outwardly waned condition in microscopic observation.
Conversely, if the deflection yoke 8 is shifted toward the electron gun 4 as shown in FIG. 8(C), the fluorescent material is put in inwardly waned condition as shown in FIG. 9(C). This adjustment of the yoke position is generally called "YPB adjustment".
Therefore, a countermeasure against doming, it has been the general practice to shift the mount position of the deflection yoke 8 slightly toward the panel 2 as shown in FIG. 8(A) to cure the symptom shown in FIG. 5(B).
In this connection, in a case where the paired compensation coils 9 are mounted over and under the deflection yoke 8 for the purpose of suppressing the leakage of magnetic fields from a CRT display, the landing condition in the horizontal direction is varied, making it difficult to attain the state of just landing as shown in FIGS. 8(B) and 9(B). Namely, in case the deflection yoke 8 is set in the position of FIG. 8(B), regardless of the compensation coils 9, the provision of the compensation coils 9 will invite the condition of FIG. 7(A).
If the deflection yoke 8 is shifted toward the panel 2 as shown in FIG. 8(A) for the purpose of overcoming the defective purity (inward waning) at the ends of X-axis, outward waning takes place at the ends of Y-axis as shown in FIG. 7(B) despite the improved purity at the ends of X-axis. Therefore, as a matter of fact, due to the difficulty of setting the position of the deflection yoke 8, there has been no choice but to take a compromising measure of setting the deflection yoke in an intermediate position between the yoke positions shown in FIGS. 7(A) and 7(B).
This difference in purity between the X-axis ends, where the beam is in the just landing condition, and the Y-axis ends, where the beam is out of the just landing condition, is generally referred to as H/V differential.
In this connection, a number of publications have been brought to our attention, including an article "Technical Movements In Mitsubishi's Large-Screen High-Quality Brown Tubes" in "TV TECHNOLOGY", pp. 17--29, June 1990, dealing with a technology for preventing deformations of the shadow mask of CRT display, and Japanese Laid-Open Patent Application H2-46085 concerning reductions of magnetic field leakage from display devices.
With a conventional CRT display of the above construction, an increase in production cost of the CRT display is inevitable in case a film of carbon graphite or bismuth oxide is coated on the inner surface of the panel 2 or shadow mask 5 or in case an Invar material of low thermal expansion is employed for the shadow mask 5 for the purpose of suppressing the doming phenomenon. Besides, there is another problem that extremely complicate meticulous skills are required to eliminate the mislanding, caused by doming or H/V differential, through adjustments of the mount position of the deflection yoke 8 in a compromising way as mentioned above.